The present invention relates to rotary regenerative air preheaters for the transfer of heat from a flue gas stream to a combustion air stream. More particularly, the present invention relates to the heat transfer surface of an air preheater.
Rotary regenerative air preheaters are commonly used to transfer heat from the flue gases exiting a furnace to the incoming combustion air. Conventional rotary regenerative air preheaters have a rotor rotatably mounted in a housing. The rotor supports heat transfer surfaces defined by heat transfer elements for the transfer of heat from the flue gases to the combustion air. The rotor has radial partitions or diaphragms defining compartments therebetween for supporting the heat transfer elements. Sector plates extend across the upper and lower faces of the rotor to divide the preheater into a gas sector and at least one air sector. The hot flue gas stream is directed through the gas sector of the preheater and transfers heat to the heat transfer elements on the continuously rotating rotor. The heated heat transfer elements are then rotated to the air sector of the preheater. The combustion air stream directed over the heat transfer elements is thereby heated.
Heat transfer elements for regenerative air preheaters have several requirements. Most importantly, the heat transfer elements must provide the required quantity of heat transfer or energy recovery for a given depth of the heat transfer element. Conventional heat transfer elements for air preheaters comprise a combination of various types of flat and/or form-pressed steel plates which are stacked in spaced relationship in heat exchange modules referred to as baskets. These spaced plates form generally longitudinal passages or channels for the flow of the flue gas stream and the air stream through the rotor. The surface design and arrangement of the heat transfer plates provides contact between adjacent plates to define and maintain the passages or channels. Further requirements for the heat transfer elements are that the stack of heat transfer elements produce minimal pressure drop for a given depth of the heat transfer elements, and furthermore, fit within a small volume.
Heat transfer element surfaces have been designed and manufactured according to many methods and geometries over the past 60 or more years. Many attempts have been made to develop new profiles which provide high levels of heat transfer with low pressure drops, and ones which are less prone to fouling, easier to clean, and not easily damaged by soot blowing. One such surface considered with excellent heat transfer and low pressure drop is shown in U.S. Pat. 4,449,573. That profile consists of a pack of heat transfer plates that are all of the same profile. The plates are provided with notches that extend obliquely to the main direction of flow. The plates are positioned such that the notches of one plate cross the notches of the second plate. The notches are parallel double ridges extending transversely from the opposite sides of a heat transfer plate. Therefore each notch forms on each surface of a heat transfer plate a peak and an immediately adjacent valley. The notches serve at least two beneficial functions, first to keep the heat transfer plates separated by a known and uniform distance. Second, the notches increase the rate of heat transfer by periodically disrupting the thermal boundary layer that forms in a flowing fluid medium over the surface of the heat transfer plate. In this manner the plates are in contact with each other only at the points spaced along the crest of the notches. While that is an improvement over the past surfaces, it does have certain disadvantages. It is difficult to clean since all particulate tends to be driven off to one side at an angle. There is no opening in the bulk direction of flow for particles, water jets or soot blowing jets. It cannot be packed loosely in a basket since the angled notches do not provide sufficient structural strength to survive the vibrations induced by soot blowing if the sheets are not snugly supported by contact with adjacent sheets. Since there is no straight line of sight through the element, an infrared or hot spot detection system is unable to detect infrared radiation at any significant depth of element. Hence there is no way to sense a hot spot condition within or downstream from the element pack.
The oblique notch described in U.S. Pat. No. 4,449,573 serves to disrupt the thermal boundary layer in the fluid and thereby increase the rate of heat transfer. In a fluid mechanics sense, the oblique notch is essentially equivalent to a uniform, periodic roughness on the surface of the plate. However, since both the plate spacing and the roughness height are proportional to the oblique notch height, it is impossible to vary the height of the roughness independently of the plate spacing. This precludes the possibility of optimizing the ratio of roughness to plate spacing. This type of optimization has been reported on in the heat transfer literature as an optimization of the ratio H/D.sub.h, where H is the roughness height and D.sub.h is the hydraulic diameter of the channel. The hydraulic diameter has units of length, and is defined as four times the ratio of the flow area divided by the wetted perimeter of the channel. For infinite, parallel flat plates, D.sub.h is equal to twice the opening between plates. For the plates of U.S. Pat. No. 4,449,573, the height of the oblique notch above the flat sheet would be H, so that the channel opening would be 2H. The D.sub.h would be approximately twice the channel opening, or 4H. This means that the ratio H/D.sub.h would always be approximately 0.25, no matter what the value of H was.
If the plate spacing could be changed independently of the roughness height, the diameter of the air preheater can be reduced so it can operate at a higher flow velocity while maintaining the same thermal recovery and pressure drop. Under these constraints, a larger plate spacing is necessary, and the result is a smaller diameter and deeper air preheater, possibly having more element weight since the larger plate spacing would typically result in lower turbulence even at the higher velocities. There are installations where this is desirable since it provides lower fouling at a higher velocity. However, with the plates of U.S. Pat. No. 4,449,573, an increased plate spacing can only be achieved by increasing the oblique notch height. At the higher velocities the higher oblique notch height produces a disproportionate pressure drop increase.